Signalling system



Feb. 28, 1967 ,J zALE$K| 3,397,168

SIGNALLING SYSTEM Filed Nov. 19, 1962 2 Sheets-Sheet a v Ja/// F X44616?lNVENTO ATTORNEY United States Iatcnt (Mike 3,307,168 SIGNALLING SYSTEMJohn F. Zaleski, Pleasantviile, N.Y., assignor to General Precision,Inc., Binghamton, N.Y., a corporation of Delaware Filed Nov. 19, 1962,Ser. No. 238,407 16 Claims. (Cl. 340-258) This invention relates tosignalling systems, and more particularly to an apparatus for signallingbetween one or more movable devices and one or more fixed locations.

Generally, in such fields as transportation and material handling, byway of example, it is often desirable to provide at a central station,information concerning the identity, location, and other characteristicsof a plurality of movable devices. These devices, including such diverseitems as railroad cars, buses, as well as bags or containers on aconveyor line, normally are routed to anyone of a number of locationsthroughout the transportation and/ or handling system.

Specifically, with respect to railway transportation systems, a train ofrailroad cars arriving at a switching terminal are first individuallyidentified and then uncoupled and sorted in a predetermined manner, withselected cars being directed to various tracks and storage locations.Further, when an outgoing train is to be assembled, it is oftennecessary that a search be made among a large number of railroad cars inorder to locate a particular car. As is well known in the prior art,various data processing systems have been deviced which are readilyadaptable to sort, classify, and store data associated with any desirednumber of cars, yet the deviation of suitable input data relating to thecharacteristics of the individual cars has proven to be relativelyexpensive, so that full use of the available data processing machineryhas not been extensive.

In order to derive the necessary input data, various prior artapparatuses have been designed, several of which are next brieflydescribed. Photoelectric sensing means have been proposed for readingbinary stripes on railroad cars, wherein the binary data is encoded ineither dark or light stripes. Additionally, microwave power has beenbeamed at passive responder units which include fractional wave lengthapertures to provide a predetermined and unique reflected signalpattern. Further, radioactive sensing means have been proposed to detectradioactive elements carried in otherwise passive responders. However,most of the systems of the prior art have, as yet, not enjoyed extensivecommercial success. This results from the fact that in many of thesystems, the accuracy and reliability of the derived input data iscritically affected by the distance between the interrogating elementand the responder element, which, of course, is movable over relativelywide limits. Also, several are drastically affected by the presence ofsuch foreign bodies as dirt, water, ice, etc. Notwithstanding the factthat some systems of the prior art have yielded acceptable performancein small scale installations, they are inherently incapable of use in asystem wherein an extremely large number of vehicles or other movableobjects are to be identified.

Yet another system of the prior art which overcomes many of thedisadvantages of the typical systems briefly described above is shown inPatent No. 3,125,753 assigned to the assignee of the present invention.As there disclosed, the system utilizes completely passive responderelements which are inductively energized as they intercept aninterrogation station, and each responder element provides a uniquelycoded response signal, when interrogated by the interrogation station,which response signal is thereafter decoded by the interrogation stationto provide the desired input data relating to the char- 3,307,168Patented Feb. 28, 1967 acteristics of the moving vehicle upon which theresponder is mounted. Apparatus of this general type exhibits severaladvantages over systems of the prior .art, including an improvedsignal-to-noise ratio which is uneffected by environmental conditions,as well as readily adaptability to large scale installations whereinmany thousands of different objects must be individually identified.

According to the present invention, however, there is provided animproved signalling system, which also includes passive responder units,wherein each responder unit provides the desired input data at amarkedly reduced cost, since filters or other complex components are'not required for each binary bit, all without affecting either thesystem reliability or the system accuracy. Briefly described, theapparatus of the invention includes one or more interrogation stationsand one or more responder elements. The interrogation station firstprovides a constant magnetic field which is effective to energize aresponder element. Each responder element, upon being energized by thismagnetic field, thereupon provides a number of signals at predeterminedradio frequencies, selected in accordance with the input data to bederived therefrom, which :are thereafter accepted by the interrogatingstation. Next, the interrogating station decodes the signals receivedfrom each energized passive responder element in a novel manner, whichincludes an electronic counter operating with a fixed time base, andprovides the input data relating to the characteristic of the movablevehicle upon which the responder elements :are mounted in a formdetermined by the overall data handling system.

It is an object of the invention, therefore, to provide an improvedsignalling system.

Another object of the invention is to provide an improved apparatus forsignalling between one or more movable devices and one or more fixedlocations.

A further object of the invention is to provide a signalling systemincluding improved passive responder elements for deriving input datarelating to the identities of or characteristics of movable vehicles. I

Still another object of the invention is to provide a more economicalsignalling system for transmitting predetermined information betweenfixed and movable locations.

Another object of the invention is to provide an improved signallingsystem for transmitting information between fixed and movable locationswherein the order of information is insensitive to the direction ofrelative motion between the locations.

Yet another object of the invention is to provide a signalling systemincluding improved means for interrogating one or more passive responderelements.

A still further object of the invention is to provide an improvedsignalling system for identifying and classifying the movable vehiclesof a transportation system.

A related object of the invention is to provide an improved signallingsystem for identifying and classifying the movable items on a conveyorline in a material handling system.

The above and other objects, features, and advantages of the inventionwill be apparent from the following detailed description of a preferredembodiment taken together with the accompanying drawings in which:

FIG. 1 is a schematic diagram of a preferred embodiment of theinvention.

FIG. 2 is a further schematic of a portion of the apparatus of FIG. 1.

FIG. 3A illustrates the magnetic field provided by a bar magnet.

FIG. 3B is an enlarged view of portions of FIG. 3A.

FIG. 4 indicates the location of a responder element and aninterrogation station as installed in one embodiment of the invention.

FIG. 5 illustrates the output signals provided by a responder element ofthe invention.

Referring now to the drawings, FIG. 1 illustrates a preferred embodimentof the invention, which includes a single responder element and aninterrogation station 12, it being understood that a plurality of suchresponder elements and one or more interrogation stations are employedin any large scale system. In general, the responder unit is mountedupon the movable device to he identified and includes a pair of passiveoscillator units, each operable at a predetermined frequency. When theresponder unit passes over or near an interrogation station, each of thepassive oscillators is sequentially automatically operated in a novelmanner, which is independent of the relative direction of motion betweena responder element and an interrogation station, as more particularlyhereinafter described.

As shown in FIG. 1, responder element 10 includes a pair of pickupinductors 13 and 14, each of which is coupled to a simple yet reliablecrystal-controlled transistor oscillator. During the time interval thatthe total flux linking inductors 13 and 14 is changing, a voltage isinduced in each inductor in the conventional manner, and this voltage issufficient to provide the necessary power for the oscillator associatedtherewith. It should be noted, however, and this is an important featureof the invention, that the oscillators are oppositely connected to theirrespective inductors, and for this reason, only one oscillator isoperable at any instant of time. By way of example, for the polarityindicated in FIG. 1, which may result from an increase in the number offlux lines linking inductors 13 and 14, only transistor 15 is suppliedwith the proper potential polarity to be operable and transistor 16 iseffectively cut-01f. Conversely, when the number of flux lines linkinginductors 13 and 14 is decreasing, the polarity of voltage induced ininductors 13 and 14 is reversed from that indicated in FIG. 1 to therebyrender transister 16 operable and render transistor 15 inoperable. Thisfeature, that one and only one oscillator is operable during aparticular time interval, will be better understood as thedescriptionproceeds.

With the polarity indicated in FIG. 1, a current flows through thecollector-emitter circuit of transistor 15 and a resonant feedback tankcircuit which includes a coil 17 and a variable capacitor 18. Inresponder element 10, coil 17 additionally functions as a loop antennato radiate the generated signal at a frequency determined by a crystal19, and the loop is distributed over as wide an area as is possible inorder to obtain maximum radiation. Additionally, a capacitor 20contributes significantly to oscillator stability and greatly increasedefiiciency at the lower crystal frequencies. However, as will beunderstood by those skilled in the art, it must be maintained at aminimum value in order to avoid excessive crystal ringing. Thedistributed capacitance of inductor 13 is effective to by-pass the RF.signal around the inductor impedance. The oscillator associated withtransistor 16 is identical to that described above, and, since theoscillators per se form no part of the invention, various other typesand circuits may be substituted therefor, provided only that they supplythe necessary stable predetermined output frequencies. Finally, a widechoice is available with respect to the inductor design other than thatas illustrated in FIG. 1, including, but not limited to, a bifilarlywound inductor, or even a single inductor with the pair of oscillatorscoupled thereto by means of a pair of oppositely-poled diodes.

Turning now to interrogation station 12 of FIG. 1, it is seen first thata pair of permanent magnets 36 and 38 are associated therewith.Alternatively, an electromagnet could also be employed. Either of thesemagnets individually provides the necessary energizing power for each ofthe oscillators of a moving responder unit, as will be better understoodin conjunction with the hereinafter detailed description of theoperation of the system. Forming .a part of interrogation station 12 isa typical receiver unit which is broadly tuned to accept any frequencygenerated by any of the plurality of responder elements. This receiveris coupled to loop antenna 39 and is represented by amplifier block 40.Block 40 provides the necessary gain to amplify the minimum signal froma responder unit to a level sulficient for use with the output circuitryemployed. Additionally, coupled to block 40 are a gate 42 and a leveldetector 55. Gate 42 isolates the output of block 40 from furtherdecoding circuitry until the level of the output exceeds a predeterminedlevel at which time detector 44 is effective to open gate 42, and tomaintain the gate open until the output signal fails to exceed thepredetermined level.

Although many and varied types of output circuits may be employed inorder to decode the frequency information received by the interrogationstation, in the preferred embodiment here being described, the decodercomprises an Events-Per-Unit-Iime Meter (or commonly an E-put meter),indicated as block 44 in FIG. 1, which is coupled to the output of gate42 by a line 46 and to detector 44 by a line 48.

A typical E-put meter comprises an electronic pulse counter effective tocount all input pulses occurring during a predetermined time period, thelength of which is controlled by an accurate and highly precise timebase. A simplified E-put meter, corresponding to block 44 of FIG. 1, isshown in somewhat more detail in FIG. 2. A crystal-controlled clockpulse oscillator 50 provides a continuous and precisely timed train ofpulses. Since the accuracy of the frequency measurement is directlydetermined by the pulse timing, this timing accuracy is generally heldto 1 part per million or less. In order to attain this precise timing,however, it is usually necessary to operate at crystal frequencies inthe order of 10 me., a frequency which does not provide for sufficientcounting time. For this reason, frequency division is accomplished inFIG. 2 by a binary counter 52 and a gate circuit 54. Gate circuit 54 isconnected to the stages of binary counter 52 in order to determine thepresence of a predetermined count therein, at which time an outputsignal is provided along a line 56. Upon the next occurrence of thepredetermined count in counter 52, another output signal is providedalong line 56. The signals along line 56 are coupled to flip-flop 58, afirst output of which is applied to the reset and readout inputs of adecimal counter 60 and the second output of which is fed to an input ofa differentiator 62.

The output of diiferentiator 62, which consists of a sequence ofalternate positive and negative accurately timed pulses, is coupled toone input of a gate 64, the operation of which is controlled by leveldetector 44 (see FIG. 1) along line 48. The output of gate 64,comprising a series of pulses, timed by flip-flop 58 when a signal abovea predetermined level is being received by interrogation station 12, isthen fed to a further flipflop 66 which controls the operation of yetanother gate 68. In this manner, signals appearing along line 46 arecoupled to counter 60 only during the predetermined time periods. Thereadout data from counter 60 is coupled to a printer or output dataconverter, the latter being effective to link the counter to a stripchart recorder, a card punch, a paper, a punch, or the like, inaccordance with the overall system requirements.

In order to illustrate the operation of the system described above, itwill be employed in an automatic railroad car identification system forpurposes of explanation only, it being understood that it is alsoadaptable to systems in other fields.

As employed in an automatic railroad car identification system, aresponder element is secured at a common loca tion of each car, with thefrequencies of the crystals therein selected in accordance with theinput data to be derived from the individual car, as more particularlyhereinafter explained. An interrogation station is then installed at adesired location with the magnets associated therewith positioned toensure that a magnetic field, of at least a predetermined magnitude, ispresent in the area to be traversed by the responder elements.Alternatively, in selected applications, the interrogation station maybe located in the movable car, with the responder elements installed inthe railroad roadbed. It should be noted, and this is an importantconsideration, that to obtain input data insensitive to the direction ofmotion of a responder element, the magnetic field provided by theinterrogation station must be parallel to the directions of motion ofthe railroad cars; i.e., parallel to the tracks, and the major axes ofinductors 13 and 14 of the responder element (see FIG. 1) must also beparallel to the tracks. Now, when a railroad car, including a responderelement, traverses an interrogation station, the responder inductorslink the flux of the magnetic field and the change in flux resultingfrom the moving inductors, and the stationary magnetic field induces avoltage in the inductors of first one polarity, for example that shownin FIG. 1, thereby energizing transistor 15, to provide a frequency fand then of the other polarity, thereby energizing transistor 16 toprovide a frequency f as hereinabove explained. This sequence of inducedpolarities which provides first a frequency A and then a frequency f isindependent of the direction of motion of the responder element past theinterrogation station, that is, the polarity sequence is the samewhether the railroad car is travelling from east to west or west toeast. Although this feature may not be readily apparent, it shouldbecome obvious from FIGS. 3A and 313.

FIG. 3A illustrates the conventional flux lines which indicate themagnetic field produced by a standard bar magnet. FIG. 3B is anenlargement of the dashed portions of FIG. 3A. Consider now a pickuploop moving from the left of FIG. 3B towards the right. This loop firstcuts fiux lines 70 thereby inducing a voltage of, by way of example,positive polarity. Next, the loop intercepts fiux lines 72, whosedirection is opposite to that of flux lines 70, and therefore a negativevoltage is induced. Now, returning the loop from the right towards theleft of FIG. 3B, the loop first cuts flux lines 72, but since its.motion is in a sense opposite to that previously described, a positivevoltage is induced, and when the loop next cuts fiux lines 70 a negativevoltage is obtained. Thus it can be seen that there is an effectivereversal of flux direction with reversal of loop motion therebyproviding the same polarity sequence independent of the motiondirection, and it is this important feature that allows the economicalresponder elements and the interrogation thereof.

Although, as described immediately above, the sequence of frequenciesgenerated by the responder element, that is, f -f is independent of thedirection of motion between the responder element and the interrogationstation, the frequency sequence is not independent of the orientation ofthe responder element with respect to the magnetic field, and it is forthis reason that a pair of separated magnets are employed in theapparatus of the invention. Referring now to FIG. 4, there isillustrated a typical installation of the apparatus as applied in arailroad car identification system. As shown, loop antenna 39 ispositioned along the center line between the tracks, while magnets 36and 38 are offset therefrom. In the general case, wherein the innerflanges of the tracks are separated by a distance of five feet, thecenter of each magnet is aligned eighteen inches inwardly from theadjacent flange, resulting in a two foot separation between the magnetsto thereby prevent the magnetic fields generated by the magnets frominteracting one to another. A responder element 10 is secured to thelower surface of a railroad car and offset one foot from the center linethereof as shown. In specific applications, wherein the movable deviceor railroad car, is movable only back and forth between a pair of fixedlocations only a single magnet is required. In the general case,however, wherein the probability of the movable device being rotatedexists, it is necessary that the double magnet installation illustratedin FIG. 4 be employed. Note should also be made of the fact that thepoles of the magnets are oppositely positioned as indicated in FIG. 4 inorder to counteract the 180 rotation of inductors 13 and 14.Alternatively, a single magnet could be employed in a generalinstallation provided that f never exceeds a predeterminer frequency andthat f never be selected to fall below the predetermined frequency. Suchrestrictions, of course, materially reduce the maximum number ofindividually identifiable responder elements.

Continuing now with the description of the system operation, bydetermining the frequencies of the combination of response signals froma given responder element when it passes an interrogation station, theidentity or other selected characteristic of the railroad car isestablished. Because of the many different combinations of systemvariables possible, such as oscillator frequencies, counting time,number of responder elements required in the system, maximum and minimumvehicle speeds, etc., only a typical system will be described in detail.

An exemplary system uses response frequencies ranging from 2.5 me. to7.5 mc. spaced at uniform 10 kc. intervals, providing 500 differentresponse frequencies. Since two crystal-controlled oscillators are usedin each responder element, approximately 250,000 different respondersmay be provided, each of which produces a unique set of two responsesignals. By way of example, employing fre quencies of 4,020 kc. and6,170 kc. in a responder, and a time base of 1 millisecond in the E-putmeter, the resulting output identification would be 4020 and 6170,identifying, for example, Car Number 402617.

It might appear at first that by reducing the channel spacing to 1 kc.an eight digit code could be realized, but the 10 kc. spacing ispreferred in order to accept the errors introduced by oscillator driftand time base tolerance.

Note should be made to the fact that the time base should not be lessthan three times the time of the shortest response signal to ensureobtaining a positive count, that is, to ensure that gate 68 (see FIG. 2)is open dur ing a counting interval, and, in general, the time base ismade longer than this.

Although a wide choice is available for each of the parameters of thesystem, the below-listed specific values are typical in a large scaleinstallation. Remembering now that the components associated withtransistor 16 of the responder element 10 shown in FIG. 1 are identicalwith those associated with transistor 15, the following table lists thevalues of a responder element oscillator:

Transistor 15 Type 2N338.

Capacitor 20 15 mmfd.

Capacitor 18 500 mmfd. max.

Resistor 21 50,000 ohms.

Inductor 13 35,000 turns, No. 37 heavy Form- Coil 18 8 rogation stationin the range of l to 60 miles per hour, the peak power level inducedinto coils l3 and 14 is at least 4 milliwatts and ensures asignal-to-noise ratio of about 30 db. Further, the operation of theresponder element is unaffected by ferrous or non-ferrous structures inthe vicinity thereof provided the structures are separated from theresponder element by at least 3 inches and do not themselves generate amagnetic field.

It should be noted that, during system operation, the oscillator has avarying impulse of voltage applied to it. Since the impedance of thetransistor is a function of this varying voltage, the crystal controlledvoltage should also be expected to vary. However, in the circuit shownand described, this variation is only 0.02% or less of the selectedfrequency. Further, over a temperature range of 50 C. to +85 C. themaximum oscillator frequency deviation is about 0.002%. This is acumulative deviation due to crystal and transistor changes. Amplitudechange over this same temperature range is less than 1 /2 decibels. Atypical response provided by a responder element is illustrated in FIG.5.

In summary then, the preferred embodiment of the invention as abovedescribed provides an automatic vehicle identification system relatingto moving vehicles, wherein the vehicle is provided with a codedresponding identification unit which is not dependent upon anyconventional built-in power supply subject to statistical failure orrequire replacement or servicing of the supply elements. The responderelement comprises a pair of radio frequency oscillators powered byinternally connected inductors which are energized as the vehiclecarrying the responder element passes over a permanent bar magnetlocated in the roadbed. The nature of the connections of the oscillatorsto the inductors is such that, regardless of the direction of travel ofthe vehicle along the roadbed the radio frequency signal combinationsare always emitted in an h-f sequence.

What has been described as an improved signalling system which includesa passive responder element and, effectively, a passive means tointerrogate the element, together with novel circuitry to decode theresponder response.

While only the fundamental novel features of the invention as applied toa preferred embodiment have been shown and described, it should beunderstood that various changes and modifications can be made withoutdeparting from the spirit of the invention, and it is the intention,therefore, only to be limited by the scope of the following claims.

What is claimed is:

1. A signalling system comprising,

at least one responder element;

at least one interrogation station;

said responder element including a pair of inductors, a pair of normallypassive oscillators, and means coupling each of said oscillators to oneof said inductors;

said interrogation station including means to provide a time-invariantmagnetic field and means responsive to response signals from said pairof normally passive oscillators of said responder element to determinethe frequencies of said oscillators;

means for providing relative motion between said responder element andsaid interrogation station whereby said magnetic field induces anoperating signal in said pair of inductors of said responder element,

said signal being effective to sequentially energize said oscillators.

2. A signalling system comprising,

(a) a responder element including first and second normally passiveoscillators, said first oscillator being operable in response to asignal of one polarity and said second oscillator being operable inresponse to a signal of the other polarity, each of said oscillatorsbeing compi ed to a pickup inductor;

(b) an interrogation station including means for providing atime-invariant magnetic field; and

(0) means for moving said responder element through a portion of saidmagnetic field in a predetermined direction to cause said inductors tosequentially provide induced signals of each of said polarities toselectively energize said first and second oscillators.

3. The system of claim 2 wherein said interrogation station includesmeasuring means to measure the frequencies of said first and secondoscillators, said measuring means including means for counting each ofsaid oscillator signals for predetermined times.

4. A signalling system comprising,

(a) an interrogation station including means for receiving andidentifying radio signals within a predetermined frequency range andmeans for providing a time-invariant magnetic field;

(b) a responder element including a pair of normally passive crystaloscillators, each of said oscillators including a resonant circuitconsisting of a coil and a capacitor wherein said coil additionallyoperates as a radiation antenna, and an inductor coupled to each of saidoscillators, each of said inductors being oppositely coupled to itsassociated oscillator; and

(c) means for providing relative motion between said interrogationstation and said responder element in a direction normal to each of apair of spaced-apart mutually-parallel oppositely-extending componentsof said magnetic field, whereby a voltage is induced in said inductors,said voltage having a pair of successive excursions operable tosequentially energize said inductors, said voltage operable tosequentially energize said oscillators to provide first and second radiosignals within said predetermined frequency range.

5. A signalling system comprising,

(a) a movable station including means selectively operable to transmitinformation signals at a plurality of discrete frequencies lying withina predetermined frequency range;

(b) a fixed station including means for receiving information signalswithin said frequency range; and

(0) means for operating said selectively operable means of said movablestation including means positioned adjacent said fixed station forproviding a predetermined time-invariant magnetic field in the path ofsaid movable station.

6. A signalling system according to claim 5 in which said means fortransmitting said information signals comprises a plurality ofsuccessively operated oscillators.

7. A signalling system according to claim 5 in which said time-invariantmagnetic field in the path of said movable station includes a pluralityof components of mutually-opposite sense whereby inductor meanstraversing said components successively will provide successiveoperating signals of opposite sense, and in which said means selectivelyoperable to transmit said information signals comprises a plurality ofoscillators connected to be selectively operated by said successiveoperating signals.

8. A signalling system according to claim 5 in which said means forreceiving said information signals includes amplifier means, leveldetector means for providing a first control signal, timing means forproviding a second control signal, a pulse counter, and gate meansresponsive to said control signals for applying signals from said'amplifier means to said pulse counter.

9. A signalling system according to claim 8 in which said timing meanscomprises a further oscillator, a counter connected to be advanced bysignals from said further oscillator, and gating means connected to beoperated by said couter to provide said second control signal.

10. A signalling system, comprising, in combination:

an interrogator device and a passive responder device,

said devices being relatively movable with respect to each other along aline in first and second opposite directions, said interrogator deviceincluding means for providing a unidirectional magnetic field having acomponent extending in a third direction normal to said line and meansfor receiving and decoding a response signal,

said responder device including an inductor operable to generate currentupon traversal of said responder device through said magnetic field, andoscillator means connected to be powered by said current to generatesaid response signal.

11. A system according to claim in which said means for receiving anddecoding said response signal comprises a pulse counter and means forapplying said response signal to said pulse counter for a predeterminedlength of time.

12. A system according to claim 10 in which said unidirectional magneticfield includes a first portion having a component extending in saidthird direction and a second portion having a component extending in afourth direction opposite to said third direction, said first and secondportions being spaced apart from each other along said line, and inwhich said oscillator means comprises a first oscillator connected toprovide response signal at a first selected frequency upon generation ofsaid current with a first polarity and a second oscillator connected toprovide response signal at a second selected frequency upon generationof said current with an opposite polarity.

13. A railroad car identification system comprising;

an interrogation station including means stationed at a railroadtrackway for providing a time-invariant magnetic field and means tomeasure the frequency of received response signals within apredetermined frequency range;

a plurality of railroad cars to be identified, each of said cars beingmovable along said trackway and each including a responder element,

each of said responder elements including normally passive oscillatormeans for transmitting to said interrogation station first and secondresponse signals when said responder elements enter into and leave fromsaid magnetic field,

the frequencies of said first and second signals as determined by saidinterrogation station providing identification indicia for each of saidplurality of railroad cars.

14. The system of claim 13 wherein said means for providing saidmagnetic field consists of a single bar magnet.

15. The system of claim 13 wherein said means for providing saidmagnetic field consists of a pair of spaced apart bar magnets.

16. A railroad car identification system comprising;

an interrogation station including means for providing a magnetic fieldhaving first and second spaced apart mutually-parallel componentsextending in first and second mutually-opposite directions, means forreceiving signals within a selected band of frequencies, and means fordetermining and indicating the frequency of any signal received withinsaid band; a plurality of railroad cars to be identified, each of saidcars including a responder element secured thereon and positioned tointercept said magnetic field when said cars pass said interrogationstation thereby to produce first and second successive operating signalsof mutually-opposite sense, the sequence in which said operating signalsare produced being independent of the direction of travel of said carwith respect to said station; each of said responder elements includingnormally passive oscillator means for transmitting to said interrogationstation first and second response signals upon generation of saidsequence of operating signals when said movable responder elementstraverse said first and second magnetic field components,

frequencies of said first and second response signals as determined bysaid interrogation station providing identification indicia for each ofsaid plurality of railroad cars.

References Cited by the Examiner UNITED STATES PATENTS 6/1962 Van Allen324-43 9/1965 Prucha 246

5. A SIGNALLING SYSTEM COMPRISING (A) A MOVABLE STATION INCLUDING MEANSSELECTIVELY OPERABLE TO TRANSMIT INFORMATION SIGNALS AT A PLURALITY OFDISCRETE FREQUENCIES LYING WITHIN A PREDETERMINED FREQUENCY RANGE; (B) AFIXED STATION INCLUDING MEANS FOR RECEIVING INFORMATION SIGNALS WITHINSAID FREQUENCY RANGE; AND (C) MEANS FOR OPERATING SAID SELECTIVELYOPERABLE MEANS OF SAID MOVABLE STATION INCLUDING MEANS POSITIONEDADJACENT SAID FIXED STATION FOR PROVIDING A PREDETERMINED TIME-INVARIANTMAGNETIC FIELD IN THE PATH OF SAID MOVABLE STATION.